As the arterial pressure wave traverses from the central aorta to the peripheral arteries, its contour becomes significantly distorted due to complex wave reflections in the distributed arterial tree. For example, both systolic (maximum) pressure and pulse pressure (systolic minus diastolic (minimum) pressure) usually become amplified, with the extent of the amplification dependent on the particular peripheral site and state of the arterial tree. Thus, it is the systolic and diastolic pressures measured specifically in the central aorta that truly reflect cardiac afterload and perfusion. Perhaps, as a result, central measurements of systolic pressure and pulse pressure have been shown to be superior in predicting patient outcome than corresponding measurements made in more peripheral arteries. Moreover, central aortic pressure is less complicated by wave reflections than peripheral artery pressure, and the entire waveform reveals each systolic ejection phase of a cardiac cycle through the dicrotic notch (which is usually obscured in peripheral artery pressure waveforms) and may be fitted to relatively simple cardiovascular models in order to accurately estimate other clinically important cardiovascular variables such as proportional cardiac output and absolute left ventricular ejection fraction. Thus, methods and apparatus for effectively monitoring the central aortic pressure waveform are extremely desirable in that they would greatly facilitate the monitoring, diagnosis, and treatment of cardiovascular disease.
The central aortic pressure waveform is conventionally measured by introducing a catheter into a peripheral artery and guiding the catheter against the flowing blood to the central aorta. However, placement of a central aortic catheter is not commonly performed in clinical practice because of the risk of blood clot formation and embolization. On the other hand, related, but distorted, peripheral artery pressure waveforms may be measured less invasively and more safely via placement of a catheter in a distal artery. Indeed, radial and femoral artery catheterizations are routinely performed in clinical practice. Moreover, over the past few decades, totally non-invasive methods have been developed and refined to continuously measure peripheral artery pressure based on finger-cuff photoplethysmography and applanation tonometry. These non-invasive methods are even available as commercial systems at present (see, for example, the Finometer and Portapres, Finapres Medical Systems, The Netherlands and the T-Line Blood Pressure Monitoring System, Tensys Medical Inc., San Diego, Calif.). In addition, non-invasive methods are commercially available for measuring signals closely related to peripheral artery pressure waveforms based on photoplethysmography.
Several techniques have therefore been recently introduced to derive the central aortic pressure waveform from related, but distorted, peripheral artery pressure waveforms. The most straightforward of the methods for deriving the central aortic pressure waveform is to measure the peripheral artery pressure waveform at a superficial artery relatively close to the heart (e.g., the carotid artery) in which the wave reflections may be small and simply use this measurement as a surrogate for the central aortic pressure waveform. However, the central aortic and carotid artery pressure waveforms have been shown to be measurably different, especially during systole. But, an even greater drawback of this method is that the carotid artery is not commonly catheterized in clinical practice due to the high level of risk and is a technically difficult site to apply applanation tonometry due to surrounding loose tissue.
Because of the practical difficulty in measuring an arterial pressure waveform relatively near the heart, several mathematical transformation methods have been developed based on a generalized transfer function approach. These methods generally involve 1) initially obtaining simultaneous measurements of central aortic and peripheral artery pressure waveforms (from, e.g., the radial artery) in a group of subjects, 2) estimating a group-averaged transfer function relating the measured peripheral artery pressure waveform to the measured central aortic pressure waveform, and 3) subsequently applying this generalized transfer function to a measured peripheral artery pressure waveform in order to predict the unobserved central aortic pressure waveform. The principal assumption underlying these methods is that arterial tree properties are constant over time and between individuals. However, the wealth of literature concerning the arterial tree indicates that this assumption is not nearly valid. For example, it is well known that the arterial compliance changes with age and disease and that the total peripheral resistance varies greatly under different physiologic conditions. As a result, the generalized transfer function approach can lead to significant discrepancies between estimated and measured central aortic pressure waveforms as well as subsequently derived indices and may be even less accurate in subjects whose measurements were not utilized in the training of the employed generalized transfer function.
A few methods have therefore been more recently developed towards “individualizing” the transfer function relating peripheral artery pressure to central aortic pressure. These methods essentially involve 1) modeling the transfer function with physiologic parameters, 2) estimating a subset of the model parameters from the peripheral artery pressure waveforms and/or other measurements from an individual while assuming values for the remaining parameters, 3) constructing a transfer function based on the estimated and assumed parameter values, and 4) applying the transfer function to the measured peripheral artery pressure waveforms to predict the corresponding central aortic pressure waveform. While these methods attempt to determine a transfer function that is specific to an individual over a particular time period, only a partial individualization is actually obtained. Perhaps, as a result, these methods have found only limited success with results not much, if at all, better than the generalized transfer function approach.
It would be desirable to have an entirely data dependent technique for determining the central aortic pressure waveform from peripheral artery pressure waveforms that is specific to the individual and time period. In this way, the central aortic pressure waveform as well as other important cardiovascular variables could be accurately and continuously monitored with minimally invasive or non-invasive measurement methods. Such a technique could be utilized for hemodynamic monitoring in the intensive care unit, operating room, and recovery room in conjunction with invasive and/or non-invasive peripheral artery pressure transducers as well as in the emergency room, at home, and in the ambulatory setting in conjunction with non-invasive peripheral artery pressure transducers.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.